1. Field of the Invention
The present invention relates to a printing apparatus for forming an image on a web that is continuously conveyed, and more particularly to a printing apparatus having a speed control mechanism for preventing transfer blurring from arising due to a variation in the web conveying speed.
2. Description of the Related Art
In the printing apparatus for forming the images on the continuous web, the web is conveyed by driving a tractor mechanism mounted on the printing apparatus with the pin members of the tractor mechanism engaged in perforations of the sheet. However, when the web with perforations was employed, there was a problem that both ends of the web with perforations must be cut out after printing, and the cutting operation was troublesome.
Therefore, a printing apparatus has been put into practical use in which the web without perforations is employed and conveyed by a conveying roller mechanism, instead of the tractor mechanism. However, in the printing apparatus for forming the image on the web without perforations, while conveying the web by the conveying roller mechanism, it is more difficult to correctly convey the web to the transfer unit when the printing speed is increased. A device for solving this problem was offered in JP-A-2001-335206.
This device includes a control mechanism for controlling the web transit position and the tension on the upstream side in a web conveying direction to the transfer unit, and a control mechanism for controlling the web transit position and the tension on the downstream side in the web conveying direction to the transfer unit to enable the web conveyance at high precision.
With the above constitution, the web is conveyed at high speed and high precision, but there is still a problem that the web is not necessarily correctly conveyed, when the conveyance load is abruptly changed, such as at the start time of printing.
That is, when the web conveyance is accelerated at the start time of printing, the load in the tension generating roller is abruptly varied. Then, the conveyance load of the web conveyance mechanism is varied, so that the web conveying speed is changed. Therefore, when the image is recorded on the web in the transfer unit, the transfer blurring arises.
Also, if the return conveyance is performed when the printing is stopped, the web is separated from the tension guide, and at the start time of next printing, an impact occurs when the web and the tension guide are contacted again, a vibration is transmitted to the transfer unit, resulting in a problem that the transfer blurring arises.
This problem is severe especially when the printing apparatus has a higher speed. That is, it has been found that the acceleration of the web conveyance speed is increased at the start time of printing, and the web tension is abruptly and excessively increased to make the transfer blurring more remarkable.